Viruses have developed strategies to counteract host defenses so as to allow viruses to infect cells and result in a latent or persistent infection. A goal of this project is to identify and determine the function of viral proteins that interact with host cell proteins to influence the course of infection. These proteins may allow us to identify new molecules that are important in the human immune system. Programmed cell death (apoptosis) is an antiviral defense mechanism used by the host to eliminate virus-infected cells. Some viruses, which cause chronic infections, encode proteins that inhibit cell death which may allow cells infected with the viruses to avoid destruction by the host's immune system. Persistent infection with hepatitis C virus (HCV) is a major cause of chronic hepatitis, cirrhosis, and liver cancer. Examination of liver biopsies from persons with chronic HCV shows that the severity of liver disease assessed by pathology correlates with the level of programmed cell death (apoptosis) in liver tissue. HCV encodes several proteins, one of which is the NS3 protein that acts both as a protease to cleave and thereby activate several viral proteins, and as a helicase to assist in virus replication. We found that expression of the HCV NS3 protein, or the NS2/NS3 precursor protein, in cell culture results in induction of programmed cell death and activation of enzymes (caspases) that mediate cell death. We found that the HCV NS3 protein bound to one of these enzymes (caspase-8) and that the activity of the HCV NS3 protein in programmed cell death could be markedly reduced by a caspase-8 enzyme inhibitor. Expression of both the HCV NS3 and the caspase-8 protein resulted in aggregation of the caspase protein in punctate structures that colocalized with the HCV NS3 protein. Analysis of mutations in the HCV NS3 protein showed that the programmed cell death activity of the viral protein is distinct from its protease and helicase activities. Another goal of this project is to identity novel viruses that cause diseases whose etiology is unknown. We are continuing to search for viruses in clinical specimens that we receive.